1. Field of the Invention
This invention relates generally to a fuel cell system and, more particularly, to a fuel cell system that employs a matched battery that eliminates the need for a DC/DC converter.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
Most fuel cell vehicles are hybrid vehicles that employ a supplemental power source in addition to the fuel cell stack, such as a DC battery or super capacitor. The power source provides supplemental power for the various vehicle auxiliary loads, for system start-up and during high power demands when the fuel cell stack is unable to provide the desired power. The fuel cell stack provides power to a traction motor through a DC voltage bus line for vehicle operation. The battery provides supplemental power to the voltage bus line during those times when additional power is needed beyond what the stack can provide, such as during heavy acceleration. For example, the fuel cell stack may provide 70 kW of power. However, vehicle acceleration may require 100 kW of power. The fuel cell stack is used to recharge the battery at those times when the fuel cell stack is able to provide the system power demand. The generator power available from the traction motor during regenerative braking is also used to recharge the battery.
In the hybrid vehicle discussed above, a bi-directional DC/DC converter is typically necessary to step up the DC voltage from the battery to match the battery voltage to the bus line voltage dictated by the voltage output of the fuel cell stack and step down the stack voltage during battery recharging. However, DC/DC converters are relatively large, costly and heavy, providing obvious disadvantages. It is desirable to eliminate the DC/DC converter from a fuel cell vehicle including a supplemental power source.
There have been various attempts in the industry to eliminate the DC/DC converter in fuel cell powered vehicles by providing a power source that is able to handle the large voltage swing coming from the fuel cell stack with its V/I characteristic (polarization curve) over the operating conditions of the vehicle. FIG. 2 is a graph with current density on the horizontal axis and fuel cell stack voltage on the vertical axis showing a typical fuel cell stack V/I characteristic or polarization curve of a stack including 400 cells in series. In one known system, a super-capacitor is used as the supplemental power source. However, the super-capacitor is limited by how much it can be discharged because of its low energy content. Also, the super-capacitor requires a power device to ramp up the super-capacitor voltage at system start-up. Certain types of batteries have also been used to eliminate the DC/DC converter in vehicle fuel cell systems. However, these systems were limited by the ability to discharge the battery beyond a certain level. In other words, these types of batteries would be damaged as a result of large voltage swings on the DC bus line during the operation of the system.